zig/test/cases/generics.zig

const assert = @import("std").debug.assert;

test "simple generic fn" {
    assert(max(i32, 3, -1) == 3);
    assert(max(f32, 0.123, 0.456) == 0.456);
    assert(add(2, 3) == 5);
}

fn max(comptime T: type, a: T, b: T) T {
    return if (a > b) a else b;
}

fn add(comptime a: i32, b: i32) i32 {
    return (comptime a) + b;
}

const the_max = max(u32, 1234, 5678);
test "compile time generic eval" {
    assert(the_max == 5678);
}

fn gimmeTheBigOne(a: u32, b: u32) u32 {
    return max(u32, a, b);
}

fn shouldCallSameInstance(a: u32, b: u32) u32 {
    return max(u32, a, b);
}

fn sameButWithFloats(a: f64, b: f64) f64 {
    return max(f64, a, b);
}

test "fn with comptime args" {
    assert(gimmeTheBigOne(1234, 5678) == 5678);
    assert(shouldCallSameInstance(34, 12) == 34);
    assert(sameButWithFloats(0.43, 0.49) == 0.49);
}

test "var params" {
    assert(max_i32(12, 34) == 34);
    assert(max_f64(1.2, 3.4) == 3.4);
}

comptime {
    assert(max_i32(12, 34) == 34);
    assert(max_f64(1.2, 3.4) == 3.4);
}

fn max_var(a: var, b: var) @typeOf(a + b) {
    return if (a > b) a else b;
}

fn max_i32(a: i32, b: i32) i32 {
    return max_var(a, b);
}

fn max_f64(a: f64, b: f64) f64 {
    return max_var(a, b);
}

pub fn List(comptime T: type) type {
    return SmallList(T, 8);
}

pub fn SmallList(comptime T: type, comptime STATIC_SIZE: usize) type {
    return struct.{
        items: []T,
        length: usize,
        prealloc_items: [STATIC_SIZE]T,
    };
}

test "function with return type type" {
    var list: List(i32) = undefined;
    var list2: List(i32) = undefined;
    list.length = 10;
    list2.length = 10;
    assert(list.prealloc_items.len == 8);
    assert(list2.prealloc_items.len == 8);
}

test "generic struct" {
    var a1 = GenNode(i32).{
        .value = 13,
        .next = null,
    };
    var b1 = GenNode(bool).{
        .value = true,
        .next = null,
    };
    assert(a1.value == 13);
    assert(a1.value == a1.getVal());
    assert(b1.getVal());
}
fn GenNode(comptime T: type) type {
    return struct.{
        value: T,
        next: ?*GenNode(T),
        fn getVal(n: *const GenNode(T)) T {
            return n.value;
        }
    };
}

test "const decls in struct" {
    assert(GenericDataThing(3).count_plus_one == 4);
}
fn GenericDataThing(comptime count: isize) type {
    return struct.{
        const count_plus_one = count + 1;
    };
}

test "use generic param in generic param" {
    assert(aGenericFn(i32, 3, 4) == 7);
}
fn aGenericFn(comptime T: type, comptime a: T, b: T) T {
    return a + b;
}

test "generic fn with implicit cast" {
    assert(getFirstByte(u8, []u8.{13}) == 13);
    assert(getFirstByte(u16, []u16.{
        0,
        13,
    }) == 0);
}
fn getByte(ptr: ?*const u8) u8 {
    return ptr.?.*;
}
fn getFirstByte(comptime T: type, mem: []const T) u8 {
    return getByte(@ptrCast(*const u8, &mem[0]));
}

const foos = []fn (var) bool.{
    foo1,
    foo2,
};

fn foo1(arg: var) bool {
    return arg;
}
fn foo2(arg: var) bool {
    return !arg;
}

test "array of generic fns" {
    assert(foos[0](true));
    assert(!foos[1](true));
}